1. Field of Invention
This invention is directed to the field of semiconductor wafer preparation and, more particularly, to an apparatus and method for transferring wafers between wafer holders.
2. Description of Related Art
During semiconductor wafer preparation, wafers are subjected to mechanical processes including slicing, grinding and lapping, which produce lattice damage in the surface regions of the wafers. This lattice damage is generally removed by chemically etching the wafers to remove the damaged surface regions of the wafers. Etching is typically conducted in an etching machine, which includes an etching drum to movably support the wafers within an acid etch solution during the etching process.
During the etching process, it is important that the wafers are positioned on the etching drum so that the wafers are parallel to, and uniformly spaced from, each other. Such positioning of the wafers is desirable so that the wafers do not contact and possibly chip or scratch each other. The wafer surfaces are very sensitive to such contact before etching (after cleaning). In addition, uniform spacing of the wafers promotes uniform etching of the wafers, so that etched wafers are uniformly sized and do not include stained regions. Non-uniform etching can produce defective wafers that are unsuitable for subsequent fabrication and must be scrapped at substantial financial cost.
It is known to manually position wafers on etching drums. The structure of etching drums makes it difficult, however, for operators to manually position the wafers on the etching drums so that the wafers are properly positioned in an efficient manner. Particularly, etching drums typically have multiple sets of grooves, with each set of grooves being designed to receive and support a single wafer. Each set of grooves typically includes several grooves that are aligned with each other. During manual positioning of the wafers, operators must position the wafers so that each of the wafers is properly received within each of the grooves of one set of grooves only. If the wafers are not properly positioned in this manner, they will be xe2x80x9ccross-indexedxe2x80x9d in the grooves, meaning that individual wafers are received in the grooves of more than one set of grooves. Consequently, more than one wafer can be received in individual grooves, causing the wafers to not all be parallel to, and equally spaced from, each other. As a result, the wafers can physically contact each other. Such contact can scratch the wafers and also prevent uniform etching of each wafer, as described above.
In order to correct cross-indexing, operators must manually re-position the wafers on the etching drum before etching. This approach is not satisfactory, however, for several reasons. First, the wafers are susceptible to scratching and, accordingly, it is desirable to minimize such handling of the wafers. Second, it is time-consuming for the operators to re-position the wafers and this additional step can significantly delay the performance of the etching step. Third, the environment of the etching machine can be hazardous to operators due to the presence of the acid etch solution and associated fumes. Correcting cross-indexing problems can force the operators to be exposed to this environment for extended periods of time.
Another concern during wafer preparation is to maintain the wafers in a clean condition. For example, wafers are very sensitive to contamination after being cleaned. Accordingly, it is important to reduce exposure of the wafers to potential contamination during transfer operations, such as by reducing exposure of cleaned wafers to the ambient atmosphere prior to etching, so as to reduce oxidation and contamination of the wafers.
Thus, there is a need for an apparatus and method for transferring wafers from wafer carriers to etching drums that can reduce or even eliminate cross-indexing, reduce exposure of operators to hazardous wafer etching environments, and reduce wafer contamination during processing and transfer operations.
This invention provides an apparatus for transferring wafers between various types of wafer holders that can satisfy the above needs.
The wafers can be semiconductor wafers as well as other types of wafers and similar shaped objects.
The apparatus can be used to transfer wafers between various types of wafer holders. For example, in some embodiments, the apparatus can transfer wafers from wafer holders that hold wafers in set relationships, such as parallel relationships, relative to each other. The apparatus can automatically transfer wafers between such wafer holders and retain the wafers in their desired relationships during transfer. The apparatus can eliminate the need for operators to manually transfer wafers between holders. In addition, the apparatus can transfer wafers without cross-indexing the wafers. Thus, the apparatus can eliminate the need for operators to manually correct cross-indexing problems in transferred wafers. Accordingly, the wafers can be prevented from contacting and damaging each other during transfer.
The apparatus can be used to transfer wafers from wafer cassettes, which are typically used to carry semiconductor wafers during wafer preparation. The apparatus can be used to automatically transfer wafers from wafer cassettes to etching drums that are utilized in etching machines. In addition, the apparatus can automatically transfer etching drums between the apparatus and etching machines, so that manual transfer is not required.
According to an embodiment of this invention, the apparatus comprises a transfer chamber for containing a liquid, and a transfer unit for transferring at least one wafer in the liquid from a first wafer holder to a second wafer holder. For example, the first and second wafer holders can be a wafer cassette and an etching drum, respectively. In some embodiments, the liquid is preferably a high-purity liquid. By transferring wafers in the liquid, the wafers can be protected against exposure to contamination during transfer.
Although not limited to these specifics, in a particular embodiment of this invention, the apparatus can comprise one or more stands disposed in the liquid in the transfer chamber. During transfer operations, the stands can support one or more of the wafers in the liquid. The apparatus can further comprise a first lift for supporting the first wafer holder and a second lift for supporting the second wafer holder. The first lift can be moved by the transfer unit to transfer one or more wafers from the first wafer holder to the first stand in the liquid. The second lift can be moved by the transfer unit to transfer one or more wafers from the second stand to the second wafer holder in the liquid.
The transfer unit can also transfer wafers in the reverse direction in the liquid, i.e., from the second wafer holder to the second stand, from the second stand to the first stand, and from the first stand to the first wafer holder. In addition, the apparatus can transfer wafers to and from other machines used for wafer fabrication, such as etching machines, that are utilized in wafer preparation. Accordingly, wafers can be transferred back to a wafer holder after the wafers are subjected to other wafer fabrication steps.
The transfer unit is preferably automated so that it can automatically transfer the wafers in the liquid between the various wafer supports. Accordingly, the apparatus can eliminate the need for operators to manually transfer wafers between wafer holders.
This invention also provides a method for transferring wafers between wafer holders in a liquid medium that utilizes the apparatus.